1. Field of the Invention
This invention relates to a method and device for controlling carrier density and contact resistance in an Organic Field Effect Transistor (OFET) or Organic Thin Film Transistor (OTFT).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in superscripts. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Solution-processed organic field-effect transistors (OFETs) based on semiconducting polymers as charge transport layers continue to attract considerable attention in both academic and industrial communities, due to their potential for mass production of flexible and cost-effective “Plastic Electronics”[1-3]. In particular, carrier mobilities have been remarkably increased over the past decades, exceeding 50 cm2 V−1s−1 (and even higher) [4]. Despite such high mobility, however, OFETs have received less attention for practical applications due to unstable device characteristics—organic semiconductors can be doped unintentionally, and thus devices often operate in the depletion mode [5, 6]. More seriously, undesired doping makes the carrier density uncontrollable, thereby leading to variable (uncontrollable) threshold voltage (VT), increased off-current (causing decreased on/off ratio), and variable carrier mobility [7]. Since constructing useful circuits requires the precise control of carrier density, one must understand how semiconducting polymers can be doped during the device fabrication process, and one must develop a facile method to achieve desired carrier density for ensuring proper operation of plastic electronic circuits.
Previous effort to control carrier density of OFETs focused mainly on surface modification of the gate dielectrics using self-assembled monolayers (SAMs) [5, 8]. By passivating the gate dielectric surface (most often silicon dioxide; SiO2) with various SAM molecules bearing different functional end groups, the carrier density and VT were varied by varying degrees of the dipole-induced built-in electric field. More recent reports describe alternative approaches by charge transfer doping of organic transport layers [9] and by introducing doped organic thin films between the dielectric and organic transport layer [10]. Although carrier density and VT were effectively controlled, these methodologies can affect the nanomorphology and resistivity of the organic semiconductors on which carrier mobility strongly depends [3]. Therefore, a reliable and facile method for controlling carrier density, while maintaining (or even increasing) the mobility of solution-processed OFETs, but without changing device configuration, needs to be developed. One or more embodiments of the present invention satisfy this need.